The Challenges of Welding Molybdenum
Molybdenum is a refractory metal with a high melting point, high strength at high temperatures, high corrosion resistance, high thermal conductivity and low resistivity, and a low coefficient of linear expansion. These characteristics make molybdenum ideal for many applications, notably those within the defense, aerospace, energy, and nuclear energy industries.
Two properties of molybdenum that greatly influence its weldability are: 1) molybdenum is hard and brittle by nature and 2) molybdenum parts can be porous as a result of the method of fabrication.
At a certain temperature, molybdenum will break in a brittle manner, rather than a ductile manner. This phenomenon, known as ductile-brittle transition, is a common characteristic of refractory metals. Ductile-brittle transition poses a challenge during welding because the material can become brittle as it cools to room temperatures due to recrystallization and/or contamination.
Molybdenum also becomes more brittle when it absorbs even a minuscule amount of oxygen or nitrogen. Therefore, it is critical that the weld piece’s exposure to oxygen after cleaning and during the welding process is minimal. When laser welding, a non-reactive shielding or cover gas is often used to protect the part by completely covering the heat affected area with the gas and forcing out any oxygen. Thorough gas coverage can be achieved by welding the part in a laser welding glove-box filled with pure gas. Another option would be to laser weld molybdenum in a vacuum, or use electron beam welding, which normally occurs in a vacuum.
Porosity issues in molybdenum parts are typically a result of how the part was originally fabricated. Molybdenum parts are often made by utilizing powder metallurgy fabrication in order to yield a fine grain structure. However, poorly done, this process can result in a high rate of micropore defects. During welding, the high-pressure gases in the micropores expand rapidly after being released into the weld pool and deteriorate the strength of the joints.